math
posted by karen .
Anny is having a party .The first guest arrived by himself.3 guests then arrive together,each successive group arrives with 2 more guests than the group that entered previously.How many guests will have arrived when the twentieth group enters the room ?

you want the sum of the 1st 12 terms of the sequence
1,3,5,7,...
Now just from experience, we know that the sum of the 1st n odd numbers is n^2, so there will be 144 people present.
Or, solving algebraically, we have an arithmetic sequence with
a=1, d=2
S12 = 12/2 (2*1 + 11*2)
= 6(2+22) = 6*24 = 144 
thanks,so what is the formula for the sum of 1st even number ?